1. Field of the Invention
The present invention relates generally to a method and apparatus for converting methanol to chemicals (MTC). More particularly, the present invention relates to an improved method and apparatus for recovering and recycling dimethylether (DME) in an MTC conversion plant.
2. Discussion of the Prior Art
Processes for making hydrocarbons from gas mixtures containing methanol, dimethylether (DME) and steam in the presence of catalysts at various temperatures and pressures are well known. U.S. Pat. No. 4,387,261 issued June 7, 1983 to Vogt et al is representative of such MTC systems, which utilize zeolite catalysts, preferably the ZSM-5 type zeolite catalysts. The primary object of the Vogt patent is the production of lower olefins (C.sub.2 -C.sub.4) in the highest possible proportions from an input feed, comprising methanol, DME and stem. As stated in Vogt, increasing the reaction pressure is in conflict with the goal of the Vogt invention, i.e., the maximization of lower olefin production. However, a problem with low pressure operation in such plants is the physical size required to obtain a desired mass flow. At a given mass flow velocity, the pumps, pipes, towers, separators, reactors, etc., of a higher pressure system will be smaller than those of a lower pressure system. Therefore, it would be more economical to operate such an MTC system at substantially higher pressures in order to occupy the least amount of plant volume possible.
Although Vogt suggests the possibility of operating the MTC reactor at pressures of up to 10 bars, the preference would be to operate at much lower pressures in order to stimulate production of lower olefins. DME is created by the dehydration of methanol and is also consumed in the conversion to hydrocarbon process. At near atmospheric pressure, the amount of DME which would be dissolved in the liquid hydrocarbon stream 7, in the figure in Vogt, is very small and can be considered insignificant. However, as the system operating pressure is raised, as much as 20% volume of the liquid hydrocarbon stream 7 will be DME at 65 psig (approximately 4 bar). Therefore, if the product separator and reactor system were to operate at above atmospheric pressures (in order to reduce the plant volume) DME recovery will be necessary in order to maintain operating efficiency and to avoid the need for further liquid hydrocarbon processing downstream of the MTC conversion plant (to remove the unwanted dissolved DME). Because DME is one of the input chemicals consumed by the MTC reactor, the recycling of all DME through the reactor, producing a zero net DME output, will provide the highest methanol-to-chemical conversion efficiency. However, it may be desirable to obtain a portion of the DME created in the system as a product.